Such a process is known from U.S. Pat. No. 4,144,278. In this known process a conjugated diolefin (or conjugated diene) is dimerized by the effect of a nitrosyl metal halide. The metal to be used is an iron triad metal, which here and hereinafter is understood to be a metal from the group formed by iron, cobalt and nickel.
Applicant has found that, when carrying out such a process, with time an unacceptable reduction of the catalytic activity occurs. This reduction is manifested in at least two ways:
1) The catalytic activity of the catalyst decreases upon storage. A freshly prepared nitrosyl metal halide catalyst has a higher activity (to be expressed in moles of converted diene per mole of catalyst and per unit of time) than a similar catalyst prepared already some time before use.
2) The catalytic activity also decreases during dimerization. Differential measurements proved that the activity is tending downwards.